Reciprocating fluid pumps are used in many industries. Reciprocating fluid pumps generally include two subject fluid chambers in a pump body. A reciprocating piston or shaft is driven back and forth within the pump body. One or more plungers (e.g., diaphragms or bellows) may be connected to the reciprocating piston or shaft. As the reciprocating piston moves in one direction, the movement of the plungers results in subject fluid being drawn into a first chamber of the two subject fluid chambers and expelled from the second chamber. As the reciprocating piston moves in the opposite direction, the movement of the plungers results in fluid being expelled from the first chamber and drawn into the second chamber. A fluid inlet and a fluid outlet may be provided in fluid communication with the first subject fluid chamber, and another fluid inlet and another fluid outlet may be provided in fluid communication with the second subject fluid chamber. The fluid inlets to the first and second subject fluid chambers may be in fluid communication with a common single pump inlet, and the fluid outlets from the first and second subject fluid chambers may be in fluid communication with a common single pump outlet, such that subject fluid may be drawn into the pump through the pump inlet from a single fluid source, and subject fluid may be expelled from the pump through a single pump outlet. Check valves may be provided at the fluid inlets and outlets to ensure that fluid can only flow into the subject fluid chambers through the fluid inlets, and fluid can only flow out of the of the subject fluid chambers through the fluid outlets.
Conventional reciprocating fluid pumps operate by shifting the reciprocating piston back and forth within the pump body. Shifting of the reciprocating piston from one direction to the other may be accomplished by using a shuttle valve, which provides drive fluid (e.g., pressurized air) to a first drive chamber associated with a first plunger and then shifts the drive fluid to a second drive chamber associated with a second plunger as the first plunger reaches a fully extended position. The shuttle valve includes a spool that shifts from a first position that directs the drive fluid to the first drive chamber to a second position that directs the drive fluid to the second drive chamber. Shifting of the shuttle valve spool may be accomplished by providing fluid communication between the drive chamber and a shift conduit when each plunger is fully extended, which enables the drive fluid to pressurize the shift conduit to shift the shuttle valve spool from one position to the other. During the rest of the pumping stroke, however, the opening to the shift conduit is kept sealed from the drive chamber to keep the shuttle valve spool from prematurely shifting and to improve the efficiency of the reciprocating fluid pump.
The opening to the shift conduit may be sealed and, at the end of each pumping stroke, unsealed from the drive chamber by use of a so-called “shift canister.” The conventional shift canister is generally cylindrical with a sealing surface on the end thereof closest to the shift conduit. The sealing surface end is integral with sidewalls of the shift canister. The interior of the shift canister is hollow for disposing an end of a shift piston therein. A shift canister cap is attached to an end of the shift canister opposite the sealing surface using, for example, threads. The shift canister cap includes a hole through which the shift piston extends. The shift canister cap has an inner diameter that is smaller than an inner diameter of the shift canister sidewalls. The shift piston includes an enlarged end that has a larger diameter than the inner diameter of the shift canister cap so that, when the plunger approaches a fully extended position, the shift piston abuts against the shift canister cap and pulls the shift canister to unseal the opening to the shift conduit.
Examples of reciprocating fluid pumps and components thereof are disclosed in, for example: U.S. Pat. No. 5,370,507, which issued Dec. 6, 1994 to Dunn et al.; U.S. Pat. No. 5,558,506, which issued Sep. 24, 1996 to Simmons et al.; U.S. Pat. No. 5,893,707, which issued Apr. 13, 1999 to Simmons et al.; U.S. Pat. No. 6,106,246, which issued Aug. 22, 2000 to Steck et al.; U.S. Pat. No. 6,295,918, which issued Oct. 2, 2001 to Simmons et al.; U.S. Pat. No. 6,685,443, which issued Feb. 3, 2004 to Simmons et al.; U.S. Pat. No. 7,458,309, which issued Dec. 2, 2008 to Simmons et al.; and U.S. Patent Application Publication No. 2010/0178184 A1, which published Jul. 15, 2010 in the name of Simmons et al. The disclosure of each of these patents and patent application is respectively incorporated herein in its entirety by this reference.
In conventional reciprocating pumps, the force required to unseal the opening of the shift conduit causes wear and even failure of the pump through breakage or deformation of the shift piston, the shift canister cap, or the shift canister. The position of the shift canister cap requires the shift piston to press directly against the shift canister cap proximate the threaded connection thereof, which may cause deformation, wear, and failure of the threaded connection. To avoid such wear or failure, the reciprocating pumps are driven at a reduced drive fluid pressure to reduce the sealing force that must be overcome to unseal the opening to the shift conduit. However, reducing the drive fluid pressure limits the rate at which subject fluid can be pumped. Additionally, conventional shift canisters may include bores longitudinally extending through the sidewalls of the shift canisters for providing fluid communication between the drive fluid chamber and the sealing surface end for directing sufficient drive fluid to the shift conduit for shifting the shuttle valve at the end of a stroke. Forming such bores takes time and resources that add to the manufacturing cost of the reciprocating pumps. Furthermore, an interface between the outer surface of the conventional shift canister and the surrounding pump body is often subject to wear and causes increased friction forces, which can further aggravate the problems described above or contribute to a separate mode of failure. Accordingly, the inventors have recognized the need for improved reciprocating pumps and associated shifting mechanisms.